The present invention relates generally to fiber optic communication, and more particularly to modulation of light for use with fiber optics in fiber optic communication schemes.
With the advent of dispersion compensating fibers, erbium doped fiber amplifiers, high speed amorphous silicon detectors, and all optical demultiplexing, fiber optic transmission speed is limited principally by the modulation speed of the optical transmitter.
High speed modulators have been invented that take advantage of the properties of superconducting materials. Superconducting materials are in there xe2x80x9csuperconductingxe2x80x9d state if the current density in the material, and the temperature of the material, and the magnetic field around the material are all below certain critical values. The critical current density (JC). critical temperature (TC), and the critical magnetic field (HC) are all dependent on the chemical composition of the material and on the presence or absence of defects and impurities. If any of these quantities rise above the critical values the material leaves its superconducting state and enters its xe2x80x9cnormalxe2x80x9d state. The material has properties similar to a semiconductor in it""s normal state and is characterized by a normal-state resistivity. The superconducting state has many of the properties of a theoretically perfect conductor. The electrical resistance is zero and electromagnetic fields are reflected by it. Thus in the superconducting state a superconducting thin film acts like a mirror with 100% reflectivity [1, 2]. In the normal state light is partially transmitted [3]. The bracketed end note reference numbers [1, 2] and all other such end notes and reference numbers cited herein appear at the end of this specification along with the reference notes themselves.
U.S. Pat. No. 5,210,637 which is incorporated herein by reference issued May 11, 1993 to Puzey for xe2x80x9cHigh Speed Light Modulationxe2x80x9d discloses a device for the high speed modulation of light wherein a layer of superconducting film is used to modulate the light. U.S. Pat. No. 5,036,042 which is incorporated herein by reference issued Jul. 30, 1991 to Hed for xe2x80x9cSwitchable Superconducting Mirrorsxe2x80x9d and discloses a device that can be used for the high speed modulation of light.
FIG. 1 herein illustrates one embodiment of U.S. Pat. No. 5,210,637 as indicated by reference numeral 10. A DC power supply 15 is connected to a light source 13 to provide constant light output. A superconducting film 14 is placed in the path of the optical output and its reflectivity is altered by a modulating circuit 16 which switches the film between its superconductivity and non-superconductive states, as described In the Puzey patent. The altered reflectivity results in optical pulses 20 which are carried away by an optical fiber 25. The superconducting film is kept cool by placing it in a dewar 22 which is cooled by means of a refrigerating device 26.
A key drawback of device 10 and the corresponding device in the Hed Patent is that they are both limited to creating optical pulses in the far infrared range (approximate wavelength of 14 microns). This is because at higher frequencies the photon energy of the light is high enough to break the binding energy of the Cooper electron pairs responsible for the phenomena of superconductivity. In order for the device to work properly the photon energy of the light must be less than the binding energy (or energy gap) of the cooper pairs. This relation is given by the formula below:
hv less than 2xcex94xe2x80x83xe2x80x83{1}
Where h is Planck""s constant, v is the frequency of light, and 2xcex94 is the energy gap of the superconductor. The energy gap of the superconductor can be found from Mattis-Bardeen [4].
2xcex94=8kTxe2x80x83xe2x80x83{2}
Where k is Boltzman""s constant, and T is the critical temperature of the superconducting material. High critical temperature Thallium compounds have critical temperatures around 128 Kelvin. Plugging this into equations {1} and {2}, the operation of the device is limited to light with a wavelength around 14 microns.
The attenuation of light in silica glass fiber (the most common material for long haul fibers) can be calculated from the formula below.
xcex1=Aexe2x88x92a/xcex+B/xcex4xe2x80x83xe2x80x83{3}
Where xcex1 is the attenuation; A, a, and B are constants that are material dependent. xcex is the wavelength. The attenuation of 14 micron light in silica glass fiber is approximately 7.32xc3x971010 dB per km using formula {3} and data from reference [5]. Therefor, applicant has found that the attenuation of 14 micron light in glass fiber is to high to be useful for telecommunication. Modern telecommunication systems are optimized for wavelengths around 1.3 or 1.55 microns and have attenuation around 0.15 dB per km. Unfortunately, light at these wavelengths (i.e. at these higher photon energies) are not compatible with the devices described in the Puzey and Hed Patents. The present invention to be described hereinafter provides a solution to this problem which has remained unsolved since as long ago as December 1988, the filing date of the ""042 Hed patent.
The present invention provides an apparatus and method for the modulation of light. A layer of superconducting material is placed in the optical path of a light source. The light source emits light with a wavelength which in a preferred embodiment is long enough that the energy of the individual photons is less than the superconducting gap of the superconductor. At least a portion of the superconducting layer is then switched between a substantially non-transparent superconducting state and a partially transparent non-superconducting state by predetermined means. The optical pulses transmitted through the portion of the superconducting layer are then converted from the original wavelength to a different wavelength, a shorter wavelength in the preferred embodiment, by a frequency converting device. The shorter wavelength in the preferred embodiment has been chosen to allow an optical fiber to efficiently carry the optical pulses without significant attenuation or dispersion.
In a specific embodiment of the present invention, the predetermined switching means is designed for switching a particular portion of the superconducting layer between its superconducting and non-superconducting states using a modulating circuit that intermittently raises the current density in the particular portion above the critical current density of the superconducting material, or raises the magnetic field of the portion above the critical field of the superconducting material, or raises the temperature of the portion above the critical temperature of the superconducting material. The frequency converting device can be a parametric amplifier, parametric oscillator, Nth harmonic generator, four wave mixer, frequency upconverter, or any other frequency converting device. Means for keeping the superconducting layer below its critical temperature can be provided by placing the device in a dewar where at least a portion of the dewar is transparent to the longer wavelength and the dewar is actively cooled by a cryogenic refrigerator.
The present invention may also include certain other features described below. The present invention can include an optical fiber, optically coupled to the frequency converting device to conduct the optical pulses away from the device, for example to a receiver, optical demultiplexer, or other useful device. The present invention can include a number of fiber optic links used to provide input data for the modulating circuit. This is useful because glass optical fibers conduct less heat than metal electrical wires. Alternatively, free space optical links may be used to provide data to the modulating circuit, eliminating a heat conducting path into the dewar all together.
It is an object of the present invention to provide an improved light modulation device which increases the range of wavelengths which can be modulated at an increased rate.
It is another object of the present invention to increase the rate at which data bits can be transmitted on a fiber optic link.
It is another object of the present invention to create high speed optical pulses that are not limited to wavelengths with photon energies lower than the superconducting gap of the superconducting material.
It is an object of the present invention to reduce the amount of heat introduced by incoming data sent to the modulating circuit.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of the preferred embodiment of the invention as illustrated in the drawings.